1. Field of the Invention
The present invention relates to a photo mask used in fabricating a semiconductor device and a method for making the photo mask. More particularly, the invention relates to a method of making an optical proximity correction (OPC) mask using an electron beam and an OPC mask manufactured using the method.
2. Description of the Related Art
In general, photo masks are an indispensable part of forming photoresist patterns. Photoresist patterns are used in the photolithography processes that are required to fabricate semiconductor devices. Such a photo mask is manufactured by forming various figures corresponding to the shape of a variety of integrated circuits on a film coated with a light-shielding material, such that the mask is selectively pervious to light. The use of the photo mask makes it possible to transfer a desired integrated circuit pattern to an exact position of a photoresist during alignment and exposure in a photolithography process.
One drawback of photo masks is that a reduction in the critical dimension of an integrated circuit or in the wavelength of a light source used for exposure results in interference patterns caused by light diffraction. When these interference patterns are present it is difficult to precisely transfer a circuit pattern with the desired critical dimension to the photoresist.
FIGS. 1A and 1B are diagrams illustrating the pattern thinning, or Kennel effect, due to interference of light during exposure. More precisely, the Kennel effect refers to how a linear pattern of a fine critical dimension is affected by the pattern density of peripheral patterns. For example, referring to FIG. 1B, although a photo mask is patterned with a desired critical dimension, when the photo mask is exposed to form a pattern on the photoresist a portion P111 of the photo mask is transferred with the desired critical dimension as shown in (b) but an isolated line pattern P122 is transferred as shown in (a) with a narrower critical dimension than desired.
To counteract the Kennel Effect, optical proximity correction (OPC) techniques have been developed to exactly transfer a pattern of a critical dimension onto a desired position. OPC techniques statistically or experimentally calculate the relationship between a desired pattern and the pattern which will actually be formed on a photoresist after performing alignment and exposure on a photo mask. Based on the calculation result, the size of the photo mask pattern is adjusted to compensate for the pattern thinning effect. As a result, an actual pattern of the desired size is formed on the photoresist. In this disclosure, a pattern made by using the OPC technique to compensate for the Kennel Effect is referred to as an amended pattern.
To compensate for the Kennel Effect in a pattern, a machine-readable file containing the amended pattern must be combined with a machine-readable file that contains the originally desired pattern, thereby forming an adjusted pattern based on the information from both of the files. That is, the desired pattern must be stored together with an additional pattern, i.e., the amended pattern, in one file, thereby substantially increasing the memory required to store the file. It is difficult for an apparatus that manufactures pattern masks with an electron beam to completely store a large capacity file in a memory unit and process the file with its central processing unit (CPU). Thus, much time is spent executing the file.